My IM client has ~50 people online (busy,available or away), but the gtalk in android shows merely around 10 contacts inclusive of contacts in busy, available or away status while using the 'Availability' option.
Setting to show all contacts will show the enormous list in Alphabetical order. Scrolling to find friends in this list is a difficult task!

It was noted that only the most popular friends are shown while using the 'Availability' option.

Is this the same issue that you observed? Try searching the contact name that you find available in IM client but not in Android. It will be shown as available.

I first suspected that Android shows only from Google+ friends list, but no. Its not even a bug. Its just that they donot have an option to show all the available list!! Instead they come up with "Recently chatted available list" and "most popular available list". Hope they add a new option or fix this.